In the production of fibrous insulating mats, especially mats formed of glass or similar mineral fibers, it is customary to initially form the mat by depositing the fibers on a perforated moving conveyor, usually with the aid of suction boxes provided under the flight of the conveyor on which the fibers are deposited. It is also customary to deposit a fiber binder on the fibers either before or during the buildup of the mat on the conveyor, such binder having adhesive characteristics and usually comprising a heat curable or hardenable material, such as a thermosetting resin, for instance, a phenol formaldehyde resin sprayed on the fibers in a solution or a suspension in a volatile liquid, such as water.
The layer of blanket of relatively loose fibers on the collecting conveyor is thereafter customarily delivered to what is commonly referred to as a mat curing oven through which the mat is fed by additional perforated conveyor means, frequently comprising a pair of endless conveyors having adjacent flights presented toward each other in spaced relation and serving to determine the thickness of the mat to be formed. Such a mat may be more or less dense, depending upon the extent of compression applied by the pair of conveyors in the mat curing oven.
During passage of the mat through the oven, the mat is subjected to heat treatment to effect curing of the fiber binder and thereby to effect stabilization of the mat at the desired thickness.
For the purpose of effecting the curing of the binder, various techniques have been employed; but quite commonly, the technique includes passage of heated air through the mat, for which purpose circulation boxes or manifolds are arranged in pairs at opposite sides of the feed path of the mat through the curing oven, such ovens quite commonly including several such pairs of circulation boxes, with provision for establishing different temperature conditions sequentially through the series of pairs, so as to regulate the curing temperature applied at different zones in the path of the mat through the curing oven.
It is a principal object of the present invention to provide not only for the heating to effect curing of the binder by the primary heating system in the general manner heretofore contemplated, but in addition, the invention contemplates employment of a second independent heating system comprising at least one pair of manifolds of relatively small size operating in relatively small localized areas at opposite sides of the path of the mat, this pair of manifolds serving to pass through the mat a heated gas having a pressure and temperature sufficiently high to raise the temperature of the core portion of the mat to a higher value than that attained in the core portion in the areas surrounding said localized areas. Moreover, the heated gas of this "secondary" binder curing system is preferably passed through the mat in a localized area located in the mid or downstream portion of the feed path so that the surface layers of the mat have already been cured and stabilized by the primary heating system. This intial stabilization of the surface layers of the mat makes possible the use of relatively high pressure in the secondary heating system without disrupting the fibers of the mat.
Although the arrangement of the invention is adaptable to the curing of a wide variety of mats and fibrous blankets, for reasons noted just above, the invention is especially advantageous in the curing of binder in relatively dense mats, because the pressure and temperature conditions employed in the secondary heating system of the present invention promote rapid penetration of the heat into the interior of even quite dense and thick fibrous products; and since the secondary high pressure air is applied after the surface layers of the mat have been stabilized, this rapid penetration is accomplished without disruption of the fibers.
In a typical installation in which the primary heat curing system involves the use of pairs of hot air circulation boxes arranged in sequence along the feed path through the oven, the invention contemplates, as a secondary heat curing system, the introduction of at least one pair of hot air circulation manifolds having relatively small localized areas lying within the zone or area of one of the pairs of boxes of the primary systems. In this installation, it is contemplated that the heated air of the secondary system operating in the localized area have a pressure higher than that of the air employed in the primary system. When employed in this configuration, the primary air circulation system serves not only to supply heat needed for the curing of the binder, but in addition, it serves also as a means for preventing escape or loss into the atmosphere of air leaking from the secondary system which operates at higher pressure.
By the employment of both primary and secondary systems, and by the employment of a higher pressure in the secondary system, the rapid penetration of the heat into the interior of the mat being cured in the localized area of the secondary manifolds, is highly effective in expediting attainment of the binder curing temperature in the interior or core portion of the mat; and it is an object of the invention to provide for rapid attainment of a binder curing temperature sufficiently high to initiate exothermic reaction of the binder resin. The attainment of such an exothermic temperature will result in continuance of the binder curing, even if succeeding zones of the curing oven are not maintained at the same elevated temperature. Therefore, in the overall curing operation, the use of the secondary high pressure system in the localized downstream curing areas effects an overall economy of the total fuel expended to accomplish the curing.
It is a further object of the invention to provide novel structural arrangements for introducing the high pressure manifolds of the secondary system in the localized areas of the circulation boxes of the primary system, these structural arrangements providing for minimization of shortcircuiting and leakage and also providing automatically for yielding of some of the shielding elements without breakage thereof, in the event of buildup of resin or other deposits on the conveyors serving to carry the mat through the curing oven.